Where does the $[\mathrm{M}/\mathrm{H}] = A\times[\mathrm{Fe}/\mathrm{H}]$ relation come from? I could not find a proper source.

This article actually says (see Eq 9) that the general relation is $\log_{10}\left(\frac{Z/X}{Z_\mathrm{sun}/X_\mathrm{sun}}\right) = [\mathrm{Fe}/\mathrm{H}]$ i.e., they seem to equate [M/H] with [Fe/H]. I assume that it is because the aforementioned A parameter is between 0.9 and 1, but again that leaves me with the need for a proper source to state that.

$\begingroup$As someone who works in the field, I have not seen your middle equation (with the $A$). [Fe/H] is often lazily conflated with [M/H], but obviously it depends on the detailed composition of the star whether that is approximately true. I suspect it is approximately true for Pop I stars, but perhaps not for older, metal poor stars which have a larger predominance of alpha elements.$\endgroup$
– Rob JeffriesApr 23 '18 at 18:33

$\begingroup$I have never seen that $A$ parameter either, I just saw it on WP an thought is sounded reasonable. Since I could not find a "proper" general definition for the relation between the $z$ metallicity and the $[Fe/H]$ iron content (aside from the one I show above, from the Bertelli article), I was not sure about its validity. You say that "[Fe/H] is often lazily conflated with [M/H]", so you agree that the A parameter sounds reasonable?$\endgroup$
– GabrielApr 23 '18 at 18:40